1. Field of the Invention
The invention generally relates to determining a charge state of a battery. More specifically, the present invention relates to determining a residual capacity of the battery using optical structures and methods. Such a determination may include a substantially instantaneous quantitative analysis of electrolyte properties within the battery, synonymous with the charge state of the battery.
2. Discussion of Related Art
Rechargeable batteries create electrical current for an externally connected electrical circuit by means of a reversible chemical reaction with an electrolyte in the battery. For example, a lead acid battery, such as a typical car battery, is known to contain water-diluted sulfuric acid as an electrolyte that chemically reacts with lead electrodes of the battery to provide electrical current for the circuit. The chemical reaction within the battery is an oxidation/reduction chemical reaction that creates the electrical current. Typically, the oxidation takes place at one electrode of the battery and the reduction takes place at another electrode of the battery.
The positive electrode of the battery is formed on a plate of lead dioxide (PbO2) which forms lead sulfate (PbSO4) on the plate when in contact with the water-diluted sulfuric acid (2H2SO4). This oxidation chemical reaction forces the positive plate to expel, or supply, electrons during this reaction, leaving the plate with a positive charge. Similarly, the negative electrode is formed on a plate composed of lead (Pb) that also creates lead sulfate when chemically reacting with the sulfuric acid. The negative plate, however, is forced to expel, or supply, positive ions during this reduction reaction, leaving the plate with a negative charge. Thus the overall chemical reaction between the electrolyte and the two electrode plates can be written as PbO2+Pb+2H2SO4=2H20+2PbSO4. Relating this equation to charge states of the battery, PbO2+Pb+2H2SO4 is synonymous with a charged battery and 2H20+2PbSO4 is synonymous with a discharged battery.
The chemical reactions of the positive and negative plates provide electrical current when the electrodes are connected to one another, such as with an application-specific electrical circuit. Once discharged, the battery is recharged for application reuse. This recharging of the battery reverses the above-mentioned chemical reaction. Similarly, when the electrolyte level does not completely submerge the plates, more electrolyte is added such that the battery may continue to properly provide electrical-current producing chemical reactions for specific applications.
The life of the battery (e.g., it's ability to properly provide these chemical reactions) depends on the battery's ability to reverse the above-mentioned chemical reaction. This ability is damaged over time, for example, when the electrolyte level is often at levels that do not completely submerge the plates or through progressive low level chemical reactions of the electrolyte with the plates, thereby causing “lead sulfation.” Lead sulfation is the chemical process in which lead sulfate crystallizes on the electrodes. The crystallization permanently damages the battery because sulfur can no longer be converted into sulfuric acid through recharging. Accordingly, an attempt to recharge a lead sulfated battery will produce no resident charge in the battery.
Prior systems attempted to address the issue of physical decreases of electrolyte levels by using optical elements. For example, the physical level of a particular electrolyte could be determined by how light was refracted through the electrolyte using a visual electrolyte level indicator. These indicators took advantage of the fact that the index of refraction of the electrolyte, regardless of electrolyte concentration, differed from that of the optical elements used. Such a visual indicator reflected light illuminating the bottom of the indicator differently based on whether the indicator was surrounded by air or liquid. From this differing reflection, one could determine when to add more electrolyte.
While these prior systems were effective at determining an amount of electrolyte within a battery, the systems did nothing to determine a charge state of the battery. However, other prior systems have been developed that do measure the charge state of the battery. These other prior systems typically exist as circuits that connect to the electrodes of the battery and measure the charge state of the battery by measuring the voltage of the battery. Such systems, while determining a charge state, actually drain the battery of electrical charge because these systems, in effect, place a charge-draining load on the battery, a process known as “load testing.” For example, in load testing, a relatively large load is applied to the battery and terminal voltage is monitored as the cells within the battery discharge. This type of charge state determination is inherently inaccurate because while the charge state is evaluated the capacity of the battery is concurrently reduced.
Still other prior systems, such as those produced by Benchmarq Microelectronics, Inc., seek to evaluate the charge state of the battery by determining how the battery accepts a charge. These “charge acceptance” systems monitor electrical current between a charger and a battery as the charger recharges the battery. Since batteries cannot be instantaneously charged, such charge acceptance systems cannot make substantially instantaneous determinations of charge states in the battery but rather determine charge acceptance over a period of time. Moreover, these systems also intrusively evaluate the charge state because they monitor the charge as it is accepted by the battery.
While some prior systems discussed herein can determine a quantity of electrolyte within a battery, none of the prior systems can determine a charge state of the battery without altering the charge state of the battery and/or impacting the life of the battery. Accordingly, it is evident that a need exists for improved methods and structures for determining charge states of batteries. Although discussed herein with respect to lead acid batteries, these problems are not specific to lead acid batteries per se as they may arise in any battery, particularly those batteries containing translucent electrolytes, such as liquid electrolytes and gel electrolytes.